Notes on Cloaking Devices (or the nearest we currently have)
As promised, some notes from last night's IET Kelvin Lecture by Prof Pendry on invisibility.
We all know that light refracts when it enters a different medium from the one it was previously passing through. How much it refracts depends on a factor called the refractive index, which itself depends on two attributes of the material, its permittivity, or degree of interaction with electric fields, and its permeability, or degree of interaction with magnetic fields.
If you vary the permittivity and permeability of a material throughout a piece of it then you will have a material with an internally varying refractive index, which means that a ray of light travelling through it will curve. Now, since these properties depend mainly on the atomic structure of a material, you might think this would be difficult. But it turns out that you can fabricate lots of what are in effect tiny little electrical circuits, each of which acts like a small piece of material with adjustable permittivity and permeability. A lot of these together form a 'metamaterial' with tailored electromagnetic properties.
What you can in effect do is design a lump of metamaterial with a hole in the middle but which rays of light pass through in such a way as to bend around the hole and smoothly line up again. This makes the hole, and anything inside it, invisible. The model used is to take a spherical volume and consider a smaller sphere within it. The inner sphere is mathematically transformed to a point, a line, or (more practically) a flat sheet.
So far doing this at optical wavelengths is not possible, but it has been done with microwaves. In that picture, the gap between the white block (in effect a microwave mirror) and the yellow metamaterial is the cloaked space. Microwaves bounce off the white block as if the gap, or anything in it, wasn't there.
The clever bit is to take the required mathematical transform used to open up the hole, and to use it to calculate the required pattern of varying permittivity and permeability of the metamaterial elements.
Other things you can do with metamaterials include making materials with a negative refractive index, which never happens in nature. These have very odd optical properties, such as focussing with theoretically infinite resolution (a 'superlens').
Pendry gave an interesting example of what negative refractive index material would look like. If you look at a milk bottle, you don't see an inner volume of milk with a clear layer of glass around it; the milk seems to fill the glass, because any light refracted into the glass ends up being refracted further into the milk. If the bottle was made of negative refractive index glass, the milk would appear to extend beyond the bottle! Similarly, if you could give water a negative refractive index, objects under the surface would look as if they were hovering above it.
My own thoughts: In SF 'cloaking device' terms, you don't want to have to embed your spaceship in a large lump of transparent metamaterial! But if you have the sort of technology that could locally change the permittivity and permeability of free space in a nonlinear manner, then presumably you would achieve the same effect and could bend light around yourself. However, turning the cloak on would not make you fade to invisibility; rather, your ship would seem to shrink to a point or flatten itself to a plane (which Pendry suggested would look like a mirror, but a mirror is hard to see in dark space).
Thanks to cthulie and cdave for coming along, and to the former for suggesting the very nice Lyceum Tavern to retire to afterwards - somewhere I must have walked past dozens of times, but have never been in.
We all know that light refracts when it enters a different medium from the one it was previously passing through. How much it refracts depends on a factor called the refractive index, which itself depends on two attributes of the material, its permittivity, or degree of interaction with electric fields, and its permeability, or degree of interaction with magnetic fields.
If you vary the permittivity and permeability of a material throughout a piece of it then you will have a material with an internally varying refractive index, which means that a ray of light travelling through it will curve. Now, since these properties depend mainly on the atomic structure of a material, you might think this would be difficult. But it turns out that you can fabricate lots of what are in effect tiny little electrical circuits, each of which acts like a small piece of material with adjustable permittivity and permeability. A lot of these together form a 'metamaterial' with tailored electromagnetic properties.
What you can in effect do is design a lump of metamaterial with a hole in the middle but which rays of light pass through in such a way as to bend around the hole and smoothly line up again. This makes the hole, and anything inside it, invisible. The model used is to take a spherical volume and consider a smaller sphere within it. The inner sphere is mathematically transformed to a point, a line, or (more practically) a flat sheet.
So far doing this at optical wavelengths is not possible, but it has been done with microwaves. In that picture, the gap between the white block (in effect a microwave mirror) and the yellow metamaterial is the cloaked space. Microwaves bounce off the white block as if the gap, or anything in it, wasn't there.
The clever bit is to take the required mathematical transform used to open up the hole, and to use it to calculate the required pattern of varying permittivity and permeability of the metamaterial elements.
Other things you can do with metamaterials include making materials with a negative refractive index, which never happens in nature. These have very odd optical properties, such as focussing with theoretically infinite resolution (a 'superlens').
Pendry gave an interesting example of what negative refractive index material would look like. If you look at a milk bottle, you don't see an inner volume of milk with a clear layer of glass around it; the milk seems to fill the glass, because any light refracted into the glass ends up being refracted further into the milk. If the bottle was made of negative refractive index glass, the milk would appear to extend beyond the bottle! Similarly, if you could give water a negative refractive index, objects under the surface would look as if they were hovering above it.
My own thoughts: In SF 'cloaking device' terms, you don't want to have to embed your spaceship in a large lump of transparent metamaterial! But if you have the sort of technology that could locally change the permittivity and permeability of free space in a nonlinear manner, then presumably you would achieve the same effect and could bend light around yourself. However, turning the cloak on would not make you fade to invisibility; rather, your ship would seem to shrink to a point or flatten itself to a plane (which Pendry suggested would look like a mirror, but a mirror is hard to see in dark space).
Thanks to cthulie and cdave for coming along, and to the former for suggesting the very nice Lyceum Tavern to retire to afterwards - somewhere I must have walked past dozens of times, but have never been in.